There have been employed rolling bearings made of high-carbon Cr steels as specified in JIS. Among these steels, SUJ2 has been used in general. SUJ2 is hardened and then tempered prior to the use such that it has a surface hardness of from about HRC60 to 63 and a retained austenite content of from about 5 to 15%.
When lubricants are contaminated with impurities, the rolling life of rolling bearings is considerably shortened, compared with the case where they are operated under clean lubrication. These impurities include metallic swarfs, shavings, flushes and wear-out powders. When rolling bearings are used under such contaminated conditions, the rolling bearing raceway and the raceway surface of races and rolling elements suffer from impressions (damage), from which flaking occurs. As a result, the life of the rolling bearings is seriously shortened.
According to "Ibutsu Konnyu Joken to Korogari Tsukare Jumyo (Contaminated conditions and rolling fatigue life)" (NSK Technical Journal No. 1555, p. 17-24, 1993), the life of rolling bearings is reduced to about 1/8, compared with the case when they are operated under clean lubrication, depending on the amount, hardness and size of impurities. This experiment is a simulation of a phenomenon that the raceway surface of a rolling bearing suffers from small impressions of several ten to several hundred .mu.m due to the contamination with impurities and flaking arises from these impressions, thus shortening the rolling fatigue, life as in the case of pitting in automotive transmission gears.
JP-B-7-110988 discloses a method wherein an appropriate amount of fine carbides are formed in the surface layer of at least one of the outer race, inner race and rolling elements of a rolling bearing to thereby improve the surface hardness and, at the same time, an appropriate amount of austenite is retained in the surface layer to thereby prevent micro-cracking under the contamination with impurities (the term "JP-B" as used herein means an "examined Japanese patent publication").
JP-A-5-255809 discloses bearing steels which contain from 0.65 to 0.90% by weight of C, from 0.15 to 0.50% by weight of Si, from 0.15 to 1.00% by weight of Mn, from 2.0 to 5.0% by weight of Cr and from 0.0090 to 0.0200% by weight of N together with from 0.010 to 0.050% of Al and/or from 0.005 to 0.50% by weight of Nb, thus lowering the C content. In these steels, the formation of a harmful white layer can be prevented by the effect of suppressing the scattering of carbon atoms attributable to mainly Cr to thereby prolong the rolling life. Moreover, the addition of Al, Nb and N contributes to the prevention of the enlargement of austenite crystal grains. The term "JP-A" as used herein means an "unexamined published Japanese patent application".
Moreover, JP-A-9-53169 discloses case hardening steels which contain from 0.1 to 0.25% of C, from 0.2 to 0.4% of Si, from 0.3 to 0.9% of Mn and from 0.5 to 0.9% of Cr, together with at least one of from 0.3 to 4.0% of Ni, from 0.01 to 0.3% of Ti, from 0.01 to 0.3% of Nb, from 0.01 to 0.3% of V and from 0.01 to 0.3% of :Zr and have a surface hardness regulated to Hv 650 to 800.
JP-A-1-255650 discloses that bearing steels which contain from 1.1 to 1.5% by weight of carbon, from 1.0 to 2.0% by weight of silicon and from 0.7 to 1.3% by weight of chromium and have a surface hardness of HRC 64 or above show elevated resisting property for temper softening and scarcely undergo plastic deformation which shortens the life.
JP-A-3-24246 discloses high carbon chromium bearing steels containing from 0.015 to 0.10% by weight of nitrogen and not more than 0.001% by weight of titanium. In these high carbon chromium bearing steels, the elevated content of nitrogen contributes to an increase in the retained austenite in the step of hardening while inhibiting the formation of TiN (titanium nitride) inclusions which are harmful to the rolling life.
In the case of the bearings of JP-A-1-255650 as described above, the matrix is reinforced and surface impressions caused by impurities, if any, are lessened by regulating the surface hardness to HRC 64 or above. When such a bearing is exposed to the invasion of hard impurities or large impressions, however, it is frequently observed that severe impressions are formed. In such a case, moreover, the high surface hardness exhibits an undesirable adverse effect of accelerating the spreading of cracking starting from the impressions. Accordingly, it cannot be expected that the life of the rolling bearings can be prolonged thereby.
In the case of the bearings of JP-A-3-24246, it is expected that the spreading of cracking can be retarded due to the high toughness of the material. However, impressions are frequently formed upon the invasion of impurities, etc. into the bearing raceway sur-face. Thus, no prolongation of the life can be expected thereby. Moreover, only titanium nitride can be formed under clean lubrication and the constitutional change occurring under high contact pressure cannot be retarded in this case. Thus, no prolonged long life can be expected thereby too.
Since alloy elements such as titanium and vanadium are merely added to the carburized and hardened parts in JP-A-9-53169, the material per se has a good impact resistance and a high toughness. However, it cannot be expected that any fine carbides can be formed thereby. When impurities invade, therefore, the raceway surface is liable to suffer from impressions. Thus, the flaking at the early stage cannot be effectively prevented in such a case.
However, the bearings disclosed by the first patent as cited above, i.e., JP-B-7-110988 are made of alloy steels containing at least from 0.3 to 0.6% by weight of C and from 3 to 14% by weight; of Cr and have been carburized or carbonitrided and hardened by heating. Thus, the heat treatment costs a great deal. In this case, moreover, the fine carbide has a large average particle diameter of 0.5 to 1.0 .mu.m and, therefore, it cannot be expected to obtain fine crystal grains.
In the bearings disclosed by the second patent, i.e., JP-A-5-255809, the carbon content is lowered and the formation of a harmful white layer is prevented by the effect of suppressing the scattering of carbon atoms attributable to mainly Cr to thereby prolong the rolling life. It is stated therein, moreover, that the addition of Al, Nb and N contributes to the prevention of the enlargement of austenite crystal grains. However, the grain size can be reduced only by about 10 in grain number merely by adding Al and Nb. Thus, it cannot be expected that the rolling life is prolonged thereby.
Since alloy elements such as Ti and V are merely added to the carburized and hardened parts in the case hardened steels disclosed in the third patent, i.e., JP-A-9-53150, the material per se has a good impact resistance and a high toughness. However, it cannot be expected that any fine carbides can be formed thereby. When impurities invade, therefore, the raceway surface is liable to suffer from impressions. Thus, the flaking at the early stage cannot be effectively prevented in such a case.
When a high contact stress is applied to a bearing, a layer hardly corroded with a corrosion liquor is formed about 100 to 300 .mu.m below the raceway surface, i.e., a constitutional change, as reported by H. Swahn et al., "Metallugical Transactions A volume 7A, August (1976), 1099-1110, Martensite Decay During Contact Fatigue in Ball Bearing". Thus, there arises a problem that this constitutional change results in flaking.
An example of the shortening of the life of a rolling bearing due to the invasion of water is reported by J. A. Cirura et al. "Wear, 24 (1973) 107-118, The Effect of Hydrogen on the Rolling Contact Fatigue Life of AISI 52100 and 440C Steel Balls". According to this report, when water is added to a lubricant in a four-point contact rolling test, the life of the rolling bearing becomes about 1/10 time shorter than the life before the addition. In a rolling fatigue test with the use of hydrogen-charged steel balls, stainless steel balls show longer life than bearing steel balls of type 2. However, it is undesirable in practice to prolong the life of bearings by using stainless steel. balls which are more expensive than bearing steel balls of type 2.